I. Field of the Invention
This invention relates generally to a fire extinguishing system in which a calculated quantity of a liquid chemical foamant is introduced into the main water stream being directed at a fire, and more particularly to a microprocessor-based control system for such equipment in which the chemical foamant is introduced into the fluid into the water stream by a positive displacement pump driven by a DC motor that is energized by a pulse width modulated signal over a first range of speeds and by a fixed-width burst-modulated pulse width modulated signal over a second range of speeds.
II. Discussion of the Prior Art
In the Haugen et al. U.S. Pat. No. 5,174,383, assigned to applicant's assignee, there is described an apparatus and method for controlling the introduction of chemical foamant into a water stream in fire-fighting equipment. That system generally comprises a water supply arranged to deliver water at varying flow rates through a hose. Associated with the hose is a flow meter which produces an electrical signal proportional to the rate of water flow through the hose. The chemical foamant is contained within a supply tank and a positive displacement piston pump having an adjustable piston stroke is driven by a variable speed electric motor for pumping the chemical foamant from the supply tank into the water stream passing through the hose. A microprocessor-based controller receives the electrical signal from the flow meter as well as another signal proportional to pump speed and then computes the rate at which the electric motor driving the pump should be driven to introduce a metered quantity of the chemical foamant into the hose such that the percentage concentration of the foamant in the water remains at a preset value even though the water flow through the hose varies.
While the described system works well to introduce prescribed quantities of chemical foamant into the water stream so as to maintain a desired ratio of chemical foamant to water, it does so at the price of requiring a pump having an adjustable stroke so that both pump speed and stroke are adjustable parameters. It had heretofore not been possible to maintain a desired percentage of chemical foamant over the entire range of flows which are encountered in fire-fighting situations without having to shut down the system so that the pump stroke could be reset.
It has been demonstrated that Class A Foam is from five to ten times more effective as a fire suppressant than water alone. Utilizing foam, fires are extinguished faster and with substantially less water damage. Foam can be an effective barrier preventing fire from spreading and protecting adjacent structures The chemical foamant, however, is not inexpensive. It is, therefore, desirable to have a foam injection system capable of automatically proportioning the foam in an exact concentration required for the specific fire-fighting problem, but without overusinq and therefore wasting the chemical foamant. In that a positive displacement pump is used to inject the concentrate into the water stream, precise control must be maintained over the pump speed as the water flow varies if a preprogrammed concentration of liquid chemical foamant to water is to be maintained. Also, the motor driving the pump must have its speed controlled over a wide range, say, from 2600 rpm down to 400 rpm.
In accordance with the present invention, a DC motor is used to drive the pump and the speed of the motor is controlled over a first predetermined range using pulse width modulation of the DC signal driving the motor. The motor will thus operate at full rated speed when the duty cycle of the pulse width modulated drive signal is 100 percent. It has been found, however, that the motor when driving the multi-cylinder, positive displacement pump load tends to stall when the duty cycle of the pulse width modulated drive signal approaches about 30 percent. This corresponds to a 3:1 duty cycle ratio which translates to a speed ratio of about 12:1. This is too small a range to adequately proportion the chemical foamant into the water stream to maintain the desired proportion over the full range of flow rates. In accordance with the prior art Haugen et al. patent referenced above, accommodation was made by having a pump whose stroke volume can be varied. The mechanism used to vary the stroke volume, however, was difficult to adjust during normal operation of the foam proportioning system.
The present invention obviates this problem by effectively extending the speed ratio of the pump from about 12:1 to about 200:1.
It is accordingly a principal object of the present invention to provide an improved foam proportioning system for fire-fighting equipment.
Another object of the invention is to provide a control arrangement which greatly extends the speed range of the pump, making it unnecessary to adjust the pump's stroke volume to cover the desired range of flow rates encountered during the use of firefighting systems.